Oh god, good question. I can never give a definitive answer, but it must be one of these:

Original Star Wars Trilogy
Ghostbusters
Big Trouble in Little China
Batman
Back to the Future
The Princess Bride
The 7th Voyage of Sinbad
Terminator 2

Those are the ones I watched in an OCD fashion when I was a single digit age. Looking at that list now, it's probably Big Trouble in Little China. I still watch that movie like 4 or 5 times a year. Minus Batman, I still love all of those movies to death, but the tale of Jack Burton has been bringing me back over and over again for 25 years.

As an adult, and discounting the old stand-by's, I think it's Batman Begins. It might also be Fargo, which somehow hits me harder and harder every time I see it. There was also a month back in 2011 where I watched The Social Network literally almost once a day.

Possibly 'Batman Begins'. I watched it three times at the cinema within it's first 24 hrs of release, and watched it constantly on dvd afterwards. Superheroes are my main fascination in life, Batman is my favourite superhero, and this was the first BM movie that delivered the character in the way I thought he should be portrayed onscreen.

But then again, I am forgetting the first X-Men movie, which was also a mucho re-watcher, due to the fact of Wolverine being portrayed onscreen perfectly, and it being the first movie to properly adapt a comic-book series in such a way as I could recognise the characters from the pages onscreen, without having to do any work in my mind to reconcile them.